It could happen anywhere in the USA --- this massacre in Russia was a test run --- also Russia's government is still in chaos and doesn't have the ability to watch for this kind of thing as well as ours. I would guess something like this will happen in a country like Mexico for similar reasons. They could easily be thinking of a takeover of that country which would give them a big long muchly unguarded border to gain access to us.
Been to East Orange lately? Thought not. Russia is still very much an authoritarian state with a ruling oligarchy. They are much better positioned to keep tabs on things than we are in this still very open and free society.
Facilitated by the politicians and teacher unions that consider it more important to keep CCW holders out of schools, than secure them against terrorists.